Due to the rapid widespread of wireless apparatuses such as mobile phones, demand for small-sized and light-weighted high-frequency filters has been increased, and an acoustic wave filter formed on a single substrate has been accepted widely in the market. This type of acoustic wave filter is formed by using FBAR (Film Acoustic Bulk Resonator), SMR (Solidly Mounted Resonator), a SAW (Surface Acoustic Wave) resonator or the like.
For example, when using signals of different passbands (frequency bands that pass the signals) just like a case of a duplexer, interference is prevented by providing an unused frequency band (guard band) between the frequency band and an adjacent frequency band. However it is required to narrow the guard band for the purpose of effective utilization of the frequency bands. For narrowing the guard band, the high-frequency filter is required to have a steep cut-off characteristic, but the cut-off characteristic is limited by the Q-value of the resonator that configures the filter.
In an acoustic wave filter proposed to provide steepness not less than the limit of Q value of the resonator, an extremum is formed at a frequency lower than the anti-resonance frequency fas of the first resonator, or an extremum is formed at a frequency higher than the resonance frequency frp of the second resonator, by use of capacitance (see Patent Document 2004-343168 for example).
However, the above-described conventional acoustic wave filters require capacitors, and use of such capacitors as external chip components hinders production of small-sized acoustic wave filters. Although it is also possible to provide a configuration where a capacitor is integrated on the same substrate on which the acoustic wave filter is provided, it requires addition of a step of manufacturing the capacitor to the process of manufacturing the acoustic wave filter, resulting in cost increase.